Which condition is characterized by the presence of a "herald patch" and subsequent widespread rash?

The condition characterized by the presence of a "herald patch" followed by a widespread rash is indeed pityriasis rosea. This skin condition typically begins with a single, larger, oval-shaped lesion known as the herald patch, which often appears on the trunk. After a few days to weeks, smaller lesions develop in a pattern that can resemble a "Christmas tree" distribution on the body.

Pityriasis rosea is thought to be viral in origin, and while the exact cause is not well understood, the herald patch is a distinctive feature that helps in diagnosis. The subsequent rash usually consists of smaller oval lesions that can be itchy, and the overall duration of the rash can last several weeks to months.

In contrast, conditions such as psoriasis, guttate psoriasis, and eczema have different presentations and causes. Psoriasis often presents with well-defined red plaques covered with thick silvery scales, while guttate psoriasis typically appears as small, drop-like lesions that may be triggered by infections. Eczema, or atopic dermatitis, usually presents with dry, itchy patches that can be chronic and recurrent but do not feature a herald patch as a characteristic sign. Thus, the unique features of pityriasis rosea, particularly the herald patch, are